Thin films of tungsten carbides deposited by reactive radio-frequency sputtering were investigated by X-ray diffraction using the Rietveld method. Two films were selected for the structural refinement. One was biased, the other unbiased. The unbiased film was found to consist of a cubic phase WC(1-x) (C(0.9)) in the space group Fm3m with a lattice parameter of 4.263 (5) angstrom. A negative substrate bias of -40 V leads to a multiphasic film: a cubic phase WC(1-x) with a lattice parameter of 4.301 (6) angstrom and a hexagonal phase W(2)C (P-3m1) with lattice parameters of a =b=2.787 (1) and c=4.549 (2) angstrom. The domain size was found to be of similar to 5 nm. The coexistence of nanocrystalline phases WC(1-x) and W(2)C is in accordance with the decrease of the carbon content (WC(0.7)) in the biased film. (C) 2008 Elsevier B.V. All rights reserved.